Most of the automotive underbody parts and the automotive body structural parts are manufactured by press-forming steel sheets having a predetermined strength at a room temperature. In recent years, from the perspective of global environmental conservation, there has been a strong demand for weight reduction of automotive bodies, and efforts have been made to reinforce steel sheets and thereby decrease the thickness of the steel sheets. However, the reinforced steel sheets have lower press-formability, and it is often difficult to press-form the steel sheets into automotive parts having desired shapes.
A technique, called a hot press-forming, is proposed in Patent Literature 1, in which both good press-formability and reinforcement of steel sheets are satisfied by hot press-forming a heated steel sheet with a mold composed of a die and a punch and simultaneously quenching the heated steel sheet. However, in the hot press-forming, the steel sheet is heated to a high temperature of approximately 950° C. before the hot press-forming, and scales (iron oxides) are formed on the surface of the steel sheet. Such scales are peeled off during the hot press-forming and causes a problem of damage to the mold or damage to the surface of hot press-formed parts.
Scales remaining on the surface of the parts also causes poor appearance and poor paint adhesiveness. Thus, the scales on the surface of the parts are generally removed by a treatment such as pickling or shot blasting. However, such a treatment makes the production process complex and decreases productivity.
Automotive underbody parts and automotive body structural parts also require excellent corrosion resistance. However, hot press-formed parts manufactured through such a treatment have no anticorrosion film, such as a coating layer, and have quite insufficient corrosion resistance.
Thus, there is a demand for a hot press-forming technique that can suppress the formation of scales during heating before hot press-forming and improve the corrosion resistance of hot press-formed parts. Therefore, a steel sheet for hot press-forming that has a film, such as a coating layer, on its surface and a hot press-forming method in which such a steel sheet is used are proposed. For example, Patent Literature 2 discloses a method for producing a hot press-formed part excellent in corrosion resistance having Zn—Fe-based or Zn—Fe—Al-based compounds on its surface by hot press-forming a steel sheet coated with Zn or Zn-based alloy.
Patent Literature 3 discloses a steel sheet for hot press-forming that includes a Ni-based coating layer and a Zn—Ni coating layer in this order on its surface. The Ni-based coating layer has a coating mass of 0.01 to 5 g/m2, and the Zn—Ni coating layer has a coating mass of 10 to 90 g/m2.